This invention pertains to a process of activating or regenerating a hydro-oxidation catalyst, preferably containing gold, or silver, or combinations thereof, dispersed on a titanium-containing support.
Hydro-oxidation catalysts find utility in hydro-oxidation processes, which involve the oxidation of organic compounds by oxygen in the presence of hydrogen. As one important use, olefins, such as propylene, can be hydro-oxidized directly with oxygen to olefin oxides, such as propylene oxide, in the presence of hydrogen and a hydro-oxidation catalyst, preferably containing gold, or silver, or combinations thereof, on a titanium-containing support. Olefin oxides, such as propylene oxide, are used inter alia to alkoxylate alcohols to form polyether polyols, such as polypropylene polyether polyols, which find significant utility in the manufacture of polyurethanes and synthetic elastomers. Another hydro-oxidation process involves the formation of useful oxygenated products, such as acetone and t-butanol, from alkanes in the presence of hydrogen, oxygen, and a hydro-oxidation catalyst.
The hydro-oxidation of olefins to olefin oxides has been described recently in several international patent publications. See, for example, WO 97/34692 and the following international patent publications of The Dow Chemical Company: WO 98/00413, WO 98/00414, WO 98/00415, which describe the use of catalysts containing gold deposited on a titanium-containing support for such processes. Likewise, international patent publication WO 99/00188 and U.S. provisional application 60/112,429, filed Dec. 16, 1998, corresponding to international patent publication WO 00/35893, also of The Dow Chemical Company, describe a catalyst containing silver or mixtures of silver and gold deposited on a titanium-containing support for the hydro-oxidation of olefins to olefin oxides. Other art, such as WO 96/02323, discloses a catalyst containing a platinum metal in at least two bond energy states deposited on titanium or vanadium silicalite for liquid phase olefin hydro-oxidations. Additional art, such as WO 97/25143, discloses a catalyst containing a lanthanide metal deposited on titanium or vanadium silicalite for liquid phase olefin hydro-oxidations. Still other art, for example EP-A1alkanes, such as propane, in the presence of hydrogen and oxygen and a hydro-oxidation catalyst to form useful oxidized products, such as acetone. Likewise, isobutane can be hydro-oxidized to t-butanol and acetone. Additional art, such as U.S. Pat. No. 5,939,569 describes a catalyst comprised of gold on zirconium-containing supports for hydro-oxidation.
A variety of catalytic supports are taught among the aforementioned references. For example, the titanium-containing supports are taught to include titanium dioxide, titanosilicates, titanium dispersed on silica (wherein the titanium exists as a disorganized phase), and likewise, titanium dispersed on certain metal silicates, as well as combinations and mixtures of the aforementioned materials. Optionally, as taught in international patent publication WO 98/00414, the catalyst can comprise one or more promoter metals selected, for example, from Group 1, Group 2, the lanthanide rare earth metals, and the actinide metals of the Periodic Table.
In a typical synthesis of the aforementioned hydro-oxidation catalysts, after the catalytic metals and optional promoter metal(s) are deposited onto the catalytic support, the composite is activated by calcining under air. or reducing under hydrogen, or by heating in an inert atmosphere, at a temperature between about 250° C. and about 800° C. for a time from about 1 hour to about 24 hours. Standard activation conditions operate at about 400° C. for 6 hours. The activated catalysts, particularly the catalysts comprising gold, silver, or combinations thereof, on a titanium-containing support, exhibit good olefin conversion and excellent selectivity to the olefin oxide, and may exhibit, depending upon the exact nature of the catalyst, long lifetime. Over time, however, these catalysts may lose some activity, and occasionally, may become sufficiently deactivated so as to render the catalyst impractical to use. At this stage of partial or full deactivation, the catalyst must be regenerated or replaced. As disclosed in WO 98/00414, for example, regeneration is taught to involve heating the deactivated catalyst for several hours under oxygen or hydrogen, optionally mixed with an inert diluent, such as, nitrogen or helium, at a temperature preferably between about 200° C. and about 400° C. Pressures ranging from atmospheric to superatmospheric can be employed. Alternatively, the deactivated catalyst can be regenerated in the presence of water, or a combination of water with oxygen or hydrogen, at similar temperatures and pressures.
The above-described activation and regeneration methods have drawbacks, first, in the length of time required, and secondly, in the high temperature required to effect the process. A high pressure may also be necessary. Typically, the activation or regeneration period consumes three to six hours. Disadvantageously, throughout this time the hydro-oxidation process is shut down. Usually, the activation and regeneration temperature significantly exceeds the hydro-oxidation process temperature, which is typically greater than about 70° C. and less than about 225° C. Accordingly, the catalyst must be heated up to the activation or regeneration temperature, and when the activation or regeneration is complete, cooled down to the operating temperature of the hydro-oxidation process. This temperature cycling consumes valuable time, during which the hydro-oxidation process remains inoperative. As an added disadvantage, the higher activation and regeneration temperature requires an input of heat and energy. Moreover, the reactor must be constructed to withstand the higher regeneration temperature and to permit cycling between the hydro-oxidation process temperature and the higher regeneration temperature. Most disadvantageously, repeated cycling through the higher activation and regeneration temperature may damage the structure of the catalyst support and/or may result in damage to the metals incorporated thereon. With each regeneration cycle a percentage of the catalyst activity may be irretrievably lost until at some point of sudden or accumulated large loss, the catalyst must be replaced.
In view of the above, it would be desirable to discover an activation and regeneration method which is efficiently accomplished in a short period of time at a temperature and pressure which are closely similar to the operating conditions of the hydro-oxidation process. Such an activation and regeneration method would reduce the heat and energy demands made on the overall process and would reduce shut-down time on the hydro-oxidation process. More desirably, since there would be essentially no recycling through higher temperature zones, the process would minimize damage to the catalytic support and damage to the catalytic metals, thereby prolonging catalyst lifetime. It would be even more desirable if the activation and regeneration method could be accomplished without introducing liquid activation or regeneration agents into the reactor, because the removal of liquids would complicate and increase the cost of the method.